How does the skin primarily regulate body temperature?

The skin primarily regulates body temperature through the process of sweating. When the body's temperature rises, sweat glands in the skin produce sweat, which is composed mostly of water and salts. As the sweat evaporates from the skin's surface, it cools the body down. This evaporative cooling is essential for maintaining a stable internal temperature, especially during physical exertion or in hot environments.

The other options do not play a significant role in the skin's ability to regulate temperature. While increased muscle activity can generate heat, it is not a direct mechanism for temperature regulation by the skin itself. Absorbing heat from sunlight can warm the skin but does not aid in cooling it down. Thickening the epidermis is a response to various environmental factors but does not directly impact the skin’s ability to regulate temperature. Thus, sweating is the most effective and primary method that the skin employs to maintain thermal homeostasis.